Field of the Invention
The present invention relates to a magnetic substance holding device, and more particularly to a magnetic substance holding device that controls magnetic fluxes from permanent magnets to thereby obtain strong holding force, to easily switch between holding and detaching, and to minimize residual magnetism.
Description of the Related Art
A magnetic substance holding device such as a permanent magnet workholding device is used to attach thereto a workpiece made of a magnetic material such as iron using magnetic force. Nowadays, such a magnetic substance holding device is widely used as an internal device attached to a mold clamping unit of an injection molding machine, a mold clamping unit of a press machine, a chuck of a machine tool, and so on.
The basic principle of such a magnetic substance holding device is that it attaches a magnetic workpiece to a holding face using strong magnetic force from a permanent magnet, and detaches the magnetic workpiece from the holding face by controlling the magnetic flux from the permanent magnet so that no magnetic flux flows through the holding face.
The method for controlling the magnetic flux from the permanent magnet may include rotating another permanent magnet which is rotatably installed to control the magnetic flux, employing an additional electromagnet to control the magnet flux, or the like.
The applicant of the present invention has already proposed a magnetic substance holding device employing an additional electromagnet (see International Publication No. WO 2012/039548). In addition, the applicant of the present invention has proposed an improved magnetic substance holding device (see Korean Patent No. 1319052).
The magnetic substance holding device disclosed in the Korean Patent No. 1319052 to the applicant of the present invention includes coils around pole pieces instead of an additional electromagnet, and accordingly has advantages in that strong holding force can be obtained in a simple structure, magnetic force from a permanent magnet can be controlled with small current at the time of switching between holding and detaching, and strong holding force can be obtained in a smaller space.
However, there is still a challenge for such a magnet substance holding device to minimize residual magnetism that attracts a workpiece even after it is detached. The magnetic substance holding devices disclosed in the above references could have reduced residual magnetism, compared to existing magnetic substance holding devices. However, in order to increase utilization of such magnetic substance holding devices, residual magnetism has to be further reduced.